With the advance of information technology such as information processing and information communications, information produced and edited in the past requires reutilization and information storage technology becomes more important. Information recording apparatuses using a variety of media such as a magnetic tape and a magnetic disk have been developed and in widespread use.
Among them, a hard-disk drive (HDD) is an auxiliary storage device of a magnetic recording method. A plurality of magnetic media as a recording medium are housed in a drive unit, and is rotated by a motor at a high speed. The medium is coated with a magnetic material such as iron oxide or cobalt chromium alloy using a plating technique or a thin-film forming technique. A magnetic head radially scans across the surface of the rotating medium and causes a magnetization in response to data on the medium to write the data, or reads the data written on the disk.
The hard disks are already in widespread use. The hard disk as a standard external storage device of a personal computer stores a diversity of computer softwares including an operating system (OS) for starting up the computer and application software programs, or stores reproduced and edited files. The hard disk drive is connected to the computer main unit through an IDE (Integrated Drive Electronics) or a SCSI (Small Computer System Interface), and the storage space of the hard disk is typically managed by a file system, such as an FAT (File Allocation Table), which is a subsystem of the host operating system.
The hard disk drives have currently a large storage capacity. With large storage capacity, the hard disk drive is used not only as a conventional computer auxiliary recording device but also as a hard disk recorder that stores audio-vidual contents broadcast and received. With the application field thereof expanded, the hard disk is used to record a variety of contents.
A physical format method of the hard disk and a data read and write operation of the hard disk, as a computer auxiliary storage device, are now considered.
The hard disk has numerous concentric “tracks” as segments for recording data. For example, track numbers 0, 1, . . . are assigned the outer most track to inner tracks. Generally, the larger the number of tracks on the surface of the hard disk, the larger the storage capacity of the medium.
Each track is divided into “sectors”, each sector serving a unit of recording. Standard read and write operations on the disk are performed on a per sector basis. The size of sector is different from medium to medium. The sector of the hard disk has typically a size of 512 bytes. Taking into consideration the utilization of the medium, outer tracks having longer track length have more sectors along to make the recording density on the track substantially uniform. This method is called a “zone bit recording” method.
The zone bit recording provides a substantially uniform recording density on the tracks while resulting in non-uniformity in data transmission speed from track to track (inner tracks presents slower data transmission speeds).
FIG. 26 diagrammatically illustrates the structure of the recording surface of the hard disk. As shown, when the hard disk drive performs an access operation, a seek operation for seeking a next track must also be performed in the case of the longest one track access.
In a hard disk drive having a plurality of media stacked in a coaxial manner, tracks of the same track number of the media are cylindrically arranged, and are thus referred to as a “cylinder.” Each cylinder is labeled the same number as the track number. For example, from the outermost side, cylinder 0, cylinder 1, . . . As a result, the track is identified by the cylinder number and a head number corresponding to the medium. A plurality of heads, each interposed between the media, are always integrally driven, moving from cylinder to cylinder.
A CHS mode is available as a method of addressing a target sector. The CHS mode is an addressing method that accesses desired data by designating a PBA (Physical Block Address) on the disk in the order of C (cylinder), H (head), and S (sector).
The CHS method is subject to a limitation in CHS parameters designated by a computer main unit which functions as a host to the hard disk drive. The CHS is unable to cope with a large-capacity hard disk. For this reason, an LBA (Logical Block Address) mode is employed. In this mode, the cylinder number, the head number, and the sector number (CHS) are expressed by a logical serial number LBA. The LBA starts with zero.
When data is read from or written to the medium in the conventional hard disk drive, the magnetic head radially scans across the disk surface to reach the track having a target sector. This action is called a “seek” operation of the magnetic head. To reach the target sector on the track, the media rotate until the target sector comes right beneath the magnetic head. This is called a “search.”
As the storage capacity of the disk becomes large, a track density of the disk increases, and a track width becomes narrow. To write data to and read data from the disk with precision, a high positioning accuracy is required of the magnetic head. A servo technique to continuously align the position of the magnetic head at the center of the track is used. A signal called a “servo pattern” is written at regular intervals on each track. The drive checks to see if the magnetic head is aligned at the center of the track (when the magnetic head passes over the servo area present on the data surface of the disk, the signal of the servo pattern is integrated to determine whether the magnetic head is on track). The servo pattern is written onto the hard disk with high precision when the hard disk drive is manufactured. FIG. 27 illustrates the servo area of the track where the servo pattern is written. Written on the servo area are a signal for positioning the head, a cylinder number, a head number, a servo number, etc.
Many conventional hard disk drives are associated with an interface such as an IDE or a SCSI for connection with a computer. In a basic disk drive control, the host computer designates a start address and the number of sectors using a command set defined by the interface.
The hard disk drive accesses sectors starting a sector designated by the address, and then continues accessing while generating a sequence for performing a read-ahead operation by predicting a sector to be accessed later.
The read-ahead operation is based on the assumption that consecutive addresses are assigned a series of data. Typically, consecutive addresses are present in consecutive head numbers or consecutive track numbers.
For example, when a large amount of data such as video data is continuously written on the media, the read-ahead operation effectively works during a read operation.
If the fragmentation of the storage area is very much in progress with a large amount of data fragmented into small pieces of data and dispersed at a plurality of locations, the read-ahead operation is not successful, designating different data during the read operation. Such an unsuccessful read-ahead operation occurs because the hard disk drive side fails to learn the structure of files handled by a host (computer main unit) requesting data read and data write.
If a prediction provided by the read-ahead operation is erroneous because of a new access request from the host, the hard disk drive seeks a track containing an address having requested data present therewithin, and waits subsequent to the completion of tracking until a target address becomes accessible. There, a seek time and a search time occur.
The read-ahead data may be stored as long as the capacity of a data buffer permits. If the prediction is erroneous consecutively or intermittently, unused old data is successively discarded in the order of old data to young data. During the read-ahead operation, the seek operation is not initiated.
The disk is subject to the seek time, the search time, the loss of time due to a delay of seek start as a result of the read-ahead operation, and data missing due to useless read-ahead operation.
The number of revolutions of the disk is increased to reduce the seek time and the search time in the ordinary disk drive. Since there is no regular pattern in the amount and structure of data used in the host such as the computer, improvements by means of the access method are impossible. Such a disk access method presents problems in power consumption and storage capacity.
As disclosed in Japanese Unexamined Patent Application Publication No. 58-33767, the use of a data buffer improves an access operation during a read operation. A read operation starts at a position which is not a start sector to which an access is requested.
As disclosed in a paper entitled “Track-aligned Extents: Matching Access Patterns to Disk Drive Characteristics” authored by Jiri Schindler et. al., Conference on FAST Jan. 28-30, 2002, Monterey, Calif., access improvements are made by allowing a host to access a disk on a track by track basis.